Today I am going to talk about the basics of a PLCs, the brain of industrial automation.
The programmable logic controller or PLC is a special-purpose computer that is hidden in the control panel of most machines.
The plc was a replacement for panels of relay devices that turn on and off machine.
The cons of relay logic control forced industry innovate PLCs. Making a machine run using relays as logic was time-consuming and challenging. Making a change in the logic or the operation of the machine was almost as complicated as starting over. Relays also fail far more frequently than computer components so that relay logic required more downtime to keep it running. Controls based on relays use a lot of electricity make lots of heat and sound and take up lots of space.
An installed PLC looks pretty simple. In appearance you have a CPU module and input/output devices referred to as IO. Sometimes the IO is part of the CPU and sometimes it is in other modules. in some equipment the i/o modules can be miles away from the CPU and connected with data cables so that the PLC is not limited to a single cabinet or even same room.
Since the plc is a computer there is no need to limit it to only digital inputs and outputs. To make these numerical devices more useful, manufacturers added analog and other numerical inputs and outputs to these devices over the years.
Plcs have calculation capability in the programming as well so that we can now calculate for instance statistical process control or SPC values and put PID controllers directly into the PLC program
As the name programmable logic controller suggest we need to program it. Most PLC’s are programmed using an application running on a PC. These specific applications communicate with the PLC using Ethernet/IP , Modbus or similar communication bus depending on the manufacturer.
Most manufacturers seem to have settled on Ethernet and/or USB in recent years but unfortunately the PLC manufacturers have failed to agree on how we program the control logic.
We have several choices when it comes to choosing a brand of plc depending on where you work you may have to familiarize yourself with many different plc brands. Some places use more than one manufacturer in a single facility so let’s list some of the major brands down so we can see the difference. Probably the biggest names out there in the automation and PLC world is Siemens, Rockwell, Allen Bradley, ABB, Omron, Mitsubishi, Schneider and Delta.